Add tipping into treasury (#4480)

* First draft

* Initial work on tests

* Add tests.

* Ensure old members can't tip.

* Fix complexity

* Update node runtime

* Build fix.

* build fix

* Fix tests

* Fix tests

* Refactor Contains impl for tests

* Introduce new way to avoid impl Contains conflicts

* Fixes

* Docs.

* Docs.

* Typo

* Whitespace

* Docs

* Typo

* Formatting

* Update frame/treasury/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/treasury/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/treasury/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Apply suggestions from code review

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Add provisional weights.

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/system/src/lib.rs

@@ -467,7 +467,7 @@ pub struct EnsureSignedBy<Who, AccountId>(sp_std::marker::PhantomData<(Who, Acco
 impl<
 	O: Into<Result<RawOrigin<AccountId>, O>> + From<RawOrigin<AccountId>>,
 	Who: Contains<AccountId>,
-	AccountId: PartialEq + Clone,
+	AccountId: PartialEq + Clone + Ord,
 > EnsureOrigin<O> for EnsureSignedBy<Who, AccountId> {
 	type Success = AccountId;
 	fn try_origin(o: O) -> Result<Self::Success, O> {